Inkjet printers operate a plurality of inkjets in each printhead to eject liquid ink onto an image receiving member. The ink can be stored in reservoirs that are located within cartridges installed in the printer. Such ink can be aqueous ink or an ink emulsion. Other inkjet printers receive ink in a solid form and then melt the solid ink to generate liquid ink for ejection onto the image receiving surface. In these solid ink printers, also known as phase change inkjet printers, the solid ink can be in the form of pellets, ink sticks, granules, pastilles, or other shapes. The solid ink pellets or ink sticks are typically placed in an ink loader and delivered through a feed chute or channel to a melting device, which melts the solid ink. The melted ink is then collected in a reservoir and supplied to one or more printheads through a conduit or the like. Other inkjet printers use gel ink. Gel ink is provided in gelatinous form, which is heated to a predetermined temperature to alter the viscosity of the ink so the ink is suitable for ejection by a printhead. Once the melted solid ink or the gel ink is ejected onto the image receiving member, the ink returns to a solid, but malleable form, in the case of melted solid ink, and to a gelatinous state, in the case of gel ink.
A typical inkjet printer uses one or more printheads with each printhead containing an array of individual nozzles through which drops of ink are ejected by inkjets across an open gap to an image receiving surface to form an ink image during printing. The image receiving surface can be the surface of a continuous web of recording media, a series of media sheets, or the surface of an image receiving member, which can be a rotating print drum or endless belt.
In an inkjet printhead, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that expel ink through an aperture, usually called a nozzle, in a faceplate of the printhead. The actuators expel an ink drop in response to an electrical signal, sometimes called a firing signal. The magnitude, or voltage level, of the firing signals affects the amount of ink ejected in an ink drop. The firing signal is generated by a printhead controller with reference to image data. A print engine in an inkjet printer processes the image data to identify the inkjets in the printheads of the printer that are operated to eject a pattern of ink drops at particular locations on the image receiving surface to form an ink image corresponding to the image data. The locations where the ink drops landed are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving surface with reference to electronic image data.
Phase change inkjet printers form images using either a direct or an offset print process. In a direct print process, melted ink is jetted directly onto recording media to form images. In an offset print process, also referred to as an indirect print process, melted ink is jetted onto a surface of a rotating member such as the surface of a rotating drum, belt, or band. Recording media are moved proximate the surface of the rotating member in synchronization with the ink images formed on the surface. The recording media are then pressed against the surface of the rotating member as the media passes through a nip formed between the rotating member and a transfix roller. The ink images are transferred and affixed to the recording media by the pressure in the nip. This process of transferring an image to the media is known as a “transfix” process.
Indirect inkjet printers are capable of producing either simplex or duplex prints. Simplex printing refers to production of an image on only one side of a print media. Duplex printing produces an image on each side of a media sheet. In duplex indirect printing, an ink image is initially formed on an intermediate drum and then transferred to the media. The media sheet is then inverted and sent along a path that passes the second side of the media sheet by the intermediate drum upon which the ink has been deposited for the formation of a second ink image on the second side.
In order for the printed images to correspond closely to the image data, both in terms of fidelity to the image objects and the colors represented by the image data, the printheads can be registered with reference to the image receiving surface and with the other printheads in the printer. Registration of printheads is a process in which the printheads eject ink in a known pattern and the printed known pattern is analyzed to determine the orientation of the printhead with respect to the imaging surface and with respect to the other printheads in the printer. The proper orientation of the printheads, however, is often verified. Additionally, verification of the proper operation of the inkjet ejectors can be made by an analysis of a printed image. The analysis generates data that is used to adjust the position of the printheads or the operation of the printheads to compensate for deviations of the printheads from the presumed conditions.
Analysis of printed images is performed with reference to two directions. “Process direction” refers to the direction in which the image receiving member is moving as the image receiving surface passes the printhead. “Cross-process direction” refers to the direction substantially perpendicular to the process direction, typically across a width of the image receiving member. In order to analyze a printed image, a test pattern is generated on the image receiving member to determine whether the actuated inkjets ejected ink and whether the ejected ink landed at a correct location, assuming the proper orientation of the printhead with the image receiving member and other printheads in the printer.
In some printing systems, a printed image is scanned with a flatbed scanner or other known offline imaging device to determine the operation of the printhead. This method of generating a picture of the printed image with an offline imaging device suffers from analyzing the printed image in situ and from inaccuracies imposed by the imaging device. In some printers, a scanner is integrated into the printer and analyzes the image while within the printer. These integrated scanners typically include one or more illumination sources and one or more optical sensors that receive radiation from the illumination source that has been reflected from the image receiving surface. Each optical sensor generates an electrical signal that corresponds to the intensity of the reflected light received by the sensor. The electrical signals from the optical sensors can be converted to digital signals by analog/digital converters and provided as digital image data to an image processor.